1. Field of the Invention
The present invention relates, in general, to a high strength aluminum alloy based on the Al—Zn—Mg—Cu alloy system and a process for forming the same. Although not limited thereto, the alloys are particularly suited for use in sporting goods and aerospace applications.
2. Description of the Background Art
The highest strength aluminum alloys known at this time are based on the aluminum-zinc-magnesium-copper system. Commercial high-strength alloys currently being produced include AA7055 (nominally 8% Zn-2% Mg-2.2% Cu-0.10% Zr), AA7068 (nominally 7.8% Zn-2.5% Mg-2.0% Cu-0.10% Zr) and a Kaiser Aluminum alloy designated K749 (nominally 8% Zn-2.2% Mg-1.8% Cu-0.14% Zr). From the published phase relationships at 860° F. for an alloy containing 8% Zn, one can note that K749 is near a phase boundary, while the other two alloys are in multiple phase fields. In the latter case all the alloying elements are not in solid solution at 860° F., and are not only unavailable for age hardening, but the undissolved phases remaining after heat treatment detract from toughness. Although solution heat treating at a higher temperature than 860° F. will dissolve more of the solute, care has to be taken to ensure that the alloy does not undergo eutectic melting, which is a common problem in commercially cast alloys that have locally enriched regions as a result of microsegregation that occurred during casting.
There is a need in many applications, such as sporting goods and aerospace applications, for even stronger alloys based on the aluminum-zinc-magnesium-copper system that do not sacrifice toughness. However, this requirement presents a problem because, in general, as the tensile strength of an aluminum alloy is increased, its toughness decreases.